The present invention relates to a reactor core used in a boiling-water reactor and to a method for operating a nuclear reactor, and more particularly to a reactor core capable of operating in a sustained manner for a long-term period, such as 15 years or longer, without fuel exchange, and to a method for operating a nuclear reactor.
The boiling-water nuclear power plants currently in use on a commercial basis have a maximum plant operating cycle of about 2 years, and at the end of each operating cycle the plants are shut down, the vessel cover of the reactor pressure vessel is opened, and a regular (periodic) inspection and fuel exchange are carried out. In this case, spent fuel is transported to and stored in a spent fuel storage pool disposed on the upper floor (section) of the reactor building, so the reactor well is filled with water pumped from a condensation storage tank or pressure suppression pool. The level of the fuel remaining in the reactor pressure vessel is adjusted in this state, and new fuel is loaded thereafter.
When the regular inspection of the plant is completed, the water in the reactor well is treated in the filter demineralizer filter or other component of a wastewater treatment system or fuel pool decontaminate system and is returned to the condensation storage tank or pressure suppression pool. These operations constitute a critical path for the steps involved in the regular inspection of a plant, and hence contribute to an increase in the number of regular inspection steps and operating steps. In addition, some of the methods for handling and managing spent fuel during its exchange and storage in spent fuel storage pools have potential problems in terms of nuclear nonproliferation.
In conventional practice, a boiling-water reactor core is configured such that a plurality of fuel assemblies, themselves obtained by arranging fuel rods and water rods in square lattices, are arranged in a square lattice at a certain pitch, and the blades of each cross-sectional cruciform control rod (cross shaped control blade in a cross section) are inserted from below into four adjacent spaces formed by the fuel assemblies facing each other. The width of a fuel assembly is about ½ foot (15.24 cm), and the width of each blade on the cruciform control rod is about 12 cm. In other words, a value of about 0.051 cm−1 is selected for the ratio (B/S) of the width (B) of each blade on a cruciform control rod (blade) and the surface area (S) of the fuel lattice defined by the surface area (S=A×A) of a square whose side is equal to the pitch (A) between the fuel assemblies.
In the conventional boiling-water reactor provided with fuel assemblies and cruciform control rods (blades) of this size, about 150 cruciform control rods (blades) are used per the reactor core per 1,000,000 kW of electric output, and some of the control rods are commonly used to reduce any excess reactivity of the nuclear reactor during operation.
Control rods (also called “control cells”) surrounded by substantially burnt-up and comparatively unreactive fuel assemblies are often used as the control rods thus introduced, and no more than about 20% of all control rods are used to adjust reactivity during operation in this case.
Excess reactivity of the conventional nuclear reactors is set at all times to about 1% Δk or more, and commonly about 2% Δk, in order to be able to continue uninterrupted operation in cases in which various variations occur during operation, plan changes are introduced, errors are made during analysis, or the like, and excess reactivity is suppressed by using the control rods as control cells during an operating cycle lasting about 1 year.
The reactivity of the boiling-water reactor is adjusted not only by techniques involving the use of the aforementioned control rods but also by techniques in which the flow rate of the reactor core is controlled by forced circulation. When the flow rate of the reactor core is controlled, the void coefficient of the reactor can be reduced or increased by increasing or reducing the flow rate of the reactor core in a corresponding manner, making it possible to increase or decrease the reactor core reactivity and to achieve finer reactivity adjustments in comparison with the use of control rods. Adjusting the flow rate of the reactor core in this manner results in a control operation in which the flow rate is subjected to a spectral shift. Therefore a flow control system is adopted as a control operating system, the circulation flow rate is reduced and the absolute value of void coefficient increased in the first half of the operating period, and the flow rate of the reactor core is increased and the void coefficient reduced thereafter.
This control operating system has the effect of suppressing reactivity by hardening the neutron energy spectrum in the first half of the operating cycle, and of enhancing neutron absorption for uranium 238 and converting the uranium 238 into plutonium. Softening the neutron energy spectrum in the second half of the operating period allows converted plutonium to function as an effective fissionable material and produces enhanced reactivity, with the result that lower uranium enrichment can be adopted for uranium 235. However, a spectral shift control operation based on the flow control is characterized in that the fuel rod of the reactor core can be adjusted within a limited range, as can the void coefficient, making it impossible to achieve markedly different void coefficients in the first and second halves of an actual operating period.
With a burnable poison, the negative reactivity worth decreases (reactivity of fuel assemblies increases) in the course of burning, and the reactivity of fuel assemblies is decreased by the reduction in fissionable materials at the burnup (burnout rate) achieved after the burnable poison has been consumed, so the variations in excess reactivity due to the burnup can, on average, be minimized for a conventional boiling-water reactor by designing it such that poison reactivity decreases exactly to zero after each operating cycle.
In addition, the concentration of a burnable poison should be adjusted depending on the burnup corresponding to a length of operating period in order to adjust the reactivity of the burnable poison. Specifically, uranium oxide and other fissionable materials (commonly ceramics) are used as a base material of burnable poisons, and uniform stabilized products thereof (so-called solid solutions) are needed for the structural stability of burnable poison pellets. Consequently, the concentration of a burnable poison cannot be raised above a certain level, and conventional boiling-water reactors are operated using burnable poisons whose concentration falls within a certain range.
Natural gadolinium has been used as a burnable poison. In addition, the control rods are made of a neutron-absorbing material, which can be obtained using boron carbide (B4C) or hafnium (Hf). Control rods made with Hf have a lower (in terms of absolute value) control rod worth (negative reactivity) in comparison with B4C, so Hf cannot be used for the entire control rod in order to achieve the same control performance as that displayed by B4C, making it necessary to adopt structures in which the ratio to water (moderator) is increased and, for example, two thin Hf sheets (thickness: about 1 mm) are combined.
There exist several needs for improving nuclear plant availability factor, economic efficiency, reduction of maintenance and improving durability. It is, however, impossible to achieve these goals with the aid of the conventional structures described above.
Because the blades of each control rod have small width per fuel assembly volume, the reactor core does not have sufficient control rod worth power operation. For this reason, the required large reduction in excess reactivity cannot be achieved despite the establishment of a much longer operating period.
Another feature of conventional fuel assembly composed of uranium fuel alone in care of long operating period is lack of reactivity at the beginning of exposure, and this reduction cannot be achieved with a burnable poison alone.
Uranium oxide or plutonium oxide is used as the nuclear fuel in the conventional boiling-water reactor, and an attempt to markedly extend the operating period without a significant increase in fuel density results in an excessively high burnup and imposes limitations on the operating period due to the corrosion of cladding of the fuel rods and the like.
Another requirement is that excess reactivity be increased in order to markedly extend the operating period, but it is impossible to establish adequate excess reactivity by conventional boiling water reactor, because their control rods have limited control capabilities.
When the boiling-water reactor is operated according to a spectral shift control operation method, its reactivity can be adjusted solely by varying the core flow of the reactor core, so the cooling capacity of the fuel assemblies is reduced when the reactor is operated at a low flow rate.
In addition, because the isotopes that have small absorbing capability are included in natural gadolinia, it is necessary to increase gadolinium concentration above the required level and creating problems in terms of mechanical stability for gadolinium-containing fuel pellets.